In ion mobility spectrometry devices, separation of gas-phase ions is accomplished by exploiting variations in ion drift velocities under an applied electric field arising from differences in ion mobilities. One well-known type of ion mobility spectrometry device is the FAIMS cell, which separates ions on the basis of a difference in the mobility of an ion at high field strength (commonly denoted as Kh) relative to the mobility of the ion at low field strength (commonly denoted as K). Briefly described, a FAIMS cell consists of a pair of spaced apart electrodes that define therebetween a separation region through which a stream of ions is directed. An asymmetric waveform comprising a high voltage component and a lower voltage component of opposite polarity, together with a DC voltage (referred to as the compensation voltage, or CV) is applied to one of the electrodes. When the ion stream contains several species of ions, only one ion species is selectively transmitted through the FAIMS cell for a given combination of asymmetric waveform peak voltage (referred to as the dispersion voltage, or DV) and CV. The remaining species of ions drift toward one of the electrode surfaces and are neutralized. The FAIMS cell may be operated in single ion detection mode, wherein the DV and CV are maintained at constant values, or alternatively the applied CV may be scanned with time to sequentially transmit ion species having different mobilities. FAIMS cells may be used for a variety of purposes, including to provide separation of an ion stream prior to entry into a mass analyzer. An example of this type of application is disclosed in U.S. Pat. No. 6,822,224 to Guevremont.
The performance of a FAIMS cell may be significantly compromised if liquid-phase material is admitted into the separation region. This condition may arise, for example, where the FAIMS cell is used in connection with an atmospheric pressure ionization source, such as an electrospray ionization source, in which a liquid solution of the analyte substance is introduced into the ionization chamber as a droplet spray. If the droplet desolvation process does not proceed to completion (which may tend to occur at high liquid flow rates), partially desolvated droplets may enter the FAIMS cell, causing several problems. First, the presence of the droplets may interfere with the separation of the ions, resulting in a loss of separation resolution (i.e., peak broadening). Second, the droplets may come into contact with the electrode surfaces, causing signal carry-over. Finally, accumulation of liquid on the electrodes will eventually cause the high-voltage asymmetric waveform to discharge, rendering the FAIMS cell inoperable.
A number of references in the prior art propose techniques for avoiding admission of liquid-phase material into the separation region of the FAIMS cell. Generally, these techniques involve providing a heated counter-flowing gas stream opposing the ion/droplet stream flow to promote desolvation of any residual droplets. Examples of this approach are described, for example, in PCT Application No. PCT/CA03/00173 (International Publication No. WO 03/067625) to Ionalytics Corporation. However, use of the counterflow gas approach carries several disadvantages. First, introduction of the counterflow gas significantly increases overall pumping requirements. Additionally, the flow rate of the counterflow gas must be carefully controlled, since inadequate or excessive flow rates can change the rate of ion transport through the FAIMS cell, in turn affecting the transmission of selected ion species. Still further, this approach may require special adaptation of one or both electrodes. Thus, there exists a need in the art for an enhanced desolvation technique for use with FAIMS cells that avoids the limitations of the counterflow gas approach.